I just bought a Robert Kaufman black cotton - three yards. I always prewash in a bathroom sink before putting things together in the washer. OMG. I must have re-placed the water 6 times before it began to be fairly clear. I had let it soak for awhile on the 2nd or 3rd "washing" and it stained my sink. Did get it clean with some sort of spray cleaner but I was pretty nervous. So I would say that you definitely want to prewash black and maybe rinse several times. If you will be putting it next to light colors, rinse a couple more.
